The present invention is generally directed to rotary drum filters, and more specifically, to disposable rotary drum filters for batch processing.
In the pharmaceutical, fine chemical, hazardous material, beverage, juice, wine, and beer industries, most of the production processes are inherently batch processes. Batch processing, rather than continuous flow processing, is advantageous when a limited volume of material is to be processed, when there is a need for traceable xe2x80x9clotsxe2x80x9d of product, and/or when the intrinsic value of the product is high. A common process step in both batch and continuous flow processing is the separation of solids from liquids.
Traditionally, solid-liquid separation for batch processing has been carried out using centrifuges, gravity filtration, vacuum filtration, and/or pressure filtration equipment. Although such equipment has been used for years, there are inherent disadvantages to its use, especially when employed in laboratory, pilot plant, or other small volume applications where time and resources are limited, and for processes in which it is critical that the materials be isolated from the outside environment.
For example, centrifuges are often relatively expensive, require significant maintenance, and are labor intensive to operate under aseptic or xe2x80x9ccleanxe2x80x9d conditions. Other types of conventional batch type filtration equipment can also be expensive and similarly require a high degree of operator attendance. A further inherent problem with prior art batch filtration units is that the designs of small batch volume filtration units are often radically different than the designs of larger batch volume equipment, making the results of scaling up from laboratory or pilot plant volume production to industrial scale production less predictable.
An additional drawback with the use of centrifuges and other batch filtration units in the laboratory and pilot plant environments is that cleaning these units between batches requires considerable time and labor. Frequently, time and technical manpower resources in laboratory and pilot plant environments are at a premium, and such cleaning operations can become an unacceptable bottleneck. These cleaning operations also introduce an element of risk, both in terms of exposing personnel to potentially hazardous materials, and in terms of introducing contaminates into the processing equipment that may degrade the quality of, or even completely ruin, the next batch to be processed.
Thus, it would be desirable to provide efficient means to separate solids and liquids that is usable in batch processing, and which does not require time consuming cleaning procedures between processing batches. To eliminate the requirement for cleaning, it would be preferable if such means were sufficiently low cost as to be disposable, so that a new unit could be used for each batch of material processed.
A common non-disposable device used for solid-liquid phase separation for continuous flow processing is the rotary drum filtration unit. In fact, the rotary drum vacuum filter is considered to be the workhorse of the chemical process industry. In this type of filter, a drum that is partially submerged in a trough of suspended solids rotates slowly at only a few revolutions per minute (RPM). Solids adhere to the outer surface of the drum, which may be coated to aid in the collection of the solids. The slow rotational speed of the drum, as well as air flow over the portion of the drum that is not submerged, help to dry the solids adhering to the drum. This solid filtrate is then removed from the rotating drum, often by a blade that scrapes the collected solids from the drum. While efficient, such units are generally quite large and expensive. Consequently, they have been used primarily in association with large-scale continuous flow processing, such as for wastewater treatment. In view of the operating efficiencies of this type of device, it would be desirable to provide a much smaller and substantially lower cost disposable filtration unit that operates according to the principles of rotary drum filtration, but which can be used for batch processing in place of the traditional centrifuges and other batch filtering units.
Preferably, such a batch filtration disposable rotary drum filter should be capable of running continuously, in a steady state fashion for a period of time sufficient to process a batch of product, enabling the device to be very small as compared to conventional types of batch equipment that would normally be employed to handle the same amount of material. It would also be desirable to provide a unit that is enclosed, to minimize the risk of product and personnel contamination. It would further be desirable to fabricate such a unit from inexpensive plastic materials that can be sterilized if a process requires that sterile equipment be used. Disposability offers the advantages of eliminating clean up, containing any hazardous materials used within the disposed device, reducing the chances of product contamination, and enhancing reliability and reproducibility.
Typical conventional rotary drum filters do not provide these desirable features. Prior art rotary drum filters are relatively complicated mechanical constructions and not readily adaptable to be mass produced in quantity and at low cost for use as a disposable unit. Specifically, prior art rotary drum filters often include complicated float valves to control the level of slurry in the trough in which the drum rotates, as well as complicated valves to control pressure in the system to ensure the proper flow of fluid into the filter unit. Because of the relatively large size of prior art rotary drum filters, and the fact that such units are designed to have very substantial service lives, such units have often incorporated relatively expensive bearing and seal components. In contrast, it would be desirable to provide a disposable rotary drum filter that replaces these mechanically complicated float control and pressure control schemes in the prior art with control means that are similarly effective, yet much simpler, lower cost, and more appropriate for a disposable unit. It would further be desirable to provide a disposable rotary drum filter that does not require expensive bearing and seal components.
Examples of prior art designs of a typical rotary drum filter include those described in U.S. Pat. Nos. 3,794,178; 3,837,499; 4,157,301; 4,346,008; 5,055,205; 5,244,572; 5,262,069; and 5,308,488. The configurations shown in these patents typically were created to optimize high production rates, to provide long service life, and with minimal concern about product loss due to retention of the product in the filters. While plastic and fiberglass have been used in the design of rotary drum filters (see U.S. Pat. Nos. 3,894,899; 4,419,165; and 4,515,693) to enable the processing of corrosive products, such rotary drum filters are not suitable for use in a single batch process as a disposable unit.
It would be therefore desirable to provide a rotary drum filter unit designed to process a batch of material, and then be replaced with a new unit for the next batch. Preferably, such a rotary drum filter would be engineered such that its fabrication costs are low and its operation is very simple. It would be desirable to provide a rotary drum filter made from plastic and encased in a plastic housing. Preferably, the unit should be relatively small, especially as compared with traditional rotary drum filters, and made from relatively inexpensive materials, such that it is practical for a single-use, and can be economically considered a disposable unit.
A desirable feature of such a single-use device would be the elimination of traditional seals and other isolation mechanisms. To further simplify the disposable unit, it would be desirable to eliminate internal prime movers and to instead rotate the inner drum using an external prime mover that is not part of the disposable component. In addition, seals associated with a drive shaft that penetrates the filter""s outer housing should preferably be eliminated by magnetic coupling of the external prime mover to an internal drive shaft. Prior art rotary filter units generally cannot employ a magnetic coupled drive, because the size of such units it too great and the magnetic coupling is incapable of providing the required torque. In contrast, a small, disposable rotary drum filter can make use of a magnetic drive because of the relatively low torque required to rotate the smaller sized device. It would further be desirable to replace the often complicated, adjustable-position metallic sharp edge of prior art rotary drum filters with a simple fixed plastic xe2x80x9csharpxe2x80x9d edge that is formed integral to the housing, to remove solids from the drum. The prior art does not disclose or suggest such an apparatus.
In accord with the present invention, a disposable rotary drum filter is defined, which is useful for separating a slurry into a liquid filtrate and a solid filter cake. The disposable rotary drum filter includes a housing defining a slurry reservoir. The housing includes a slurry inlet adapted to receive a slurry into the slurry reservoir, a gas inlet adapted to receive gas into the housing, a filtrate outlet adapted to convey filtrate from the housing after the filtrate has been separated from the filter cake, a filter cake outlet adapted to convey the filter cake from the housing, and an overflow outlet. The overflow outlet is disposed within the housing and has an opening disposed at an elevation selected to determine a maximum level of a slurry within the slurry reservoir. An inner drum is rotatably mounted within the housing and includes a generally cylindrical frame. In addition, the inner drum includes a filter extending around the frame and is disposed within the housing such that the filter is partially below the level determined by the opening of the overflow outlet. The filter is adapted to receive a filter cake that is deposited on an outer surface of the filter as filtrate separated from the filter cake is drawn through the filter and the filtrate outlet. A portion of the frame provides fluid communication between an interior surface of the filter and the filtrate outlet and is thereby adapted to convey the filtrate from the inner drum after the filtrate has passed though said filter. A driven member is connected to the frame and is adapted to be drivingly coupled to an external prime mover to rotate the inner drum, exposing the filter cake deposited on the filter to gas that is drawn through the filter into the inner drum from inside the housing. This flow of gas through the filter cake at least partially dries it. Also included within the device are means for removing at least a portion of the filter cake deposited on said filter after the filter cake has been at least partially dried by the gas. The filter cake that is thus removed passes through said filter cake outlet.
The slurry inlet is preferably disposed at a bottom of the housing and adapted to couple to a fluid line that conveys the slurry from an outlet of a slurry pump. Also, the slurry reservoir is preferably disposed in a lower portion of the housing, such that said inner drum is partially disposed within the slurry reservoir.
Also included in the housing is a journal that supports the inner drum so as to enable the internal drum to freely rotate about its longitudinal axis. A spring is provided to exert a biasing force to position the inner drum at a desired location along its longitudinal axis, within the housing. This spring is preferably disposed between the driven member and the housing. In one embodiment, the driven member is magnetically coupled to a driving member, and a magnetic force is used to provide a biasing force to similarly position the inner drum at a desired location along its longitudinal axis, within the housing, thus eliminating the need for a spring.
In one preferred embodiment, the filter cake outlet is oriented vertically, such that a gravitational force facilitates the removal of the filter cake from the housing. In this embodiment, the filter cake outlet is elongate and is disposed along one side of the housing, extending generally in alignment with the outer surface of the filter.
The means for removing at least the portion of the filter cake preferably comprises an elongate edge formed in the housing. This edge extends adjacent to the outer surface of the filter.
The filtrate outlet is preferably adapted to couple to a fluid line connected with a vacuum source.
The filter may comprise a plastic mesh screen having a plurality of openings sized to enable the filtrate to pass therethrough. However, the openings are sized to block passage of the solids comprising the filter cake. In one preferred embodiment, the filter is fabricated from a porous plastic material, whose openings are similarly sized to block passage of the solids.
In a preferred embodiment, the gas inlet is disposed in an upper part of the housing. Pressure control means are included for equalizing a pressure drop within the housing as the gas within the housing is drawn into the inner drum through the filter. The pressure control means include an orifice through which gas flowing through the filtrate outlet is drawn. This orifice is preferably formed in a plate disposed adjacent to the filtrate outlet. A fluid flow through the filter is optimized by selecting an appropriate size of the orifice in the plate.
A plurality of fluid channels in fluid communication with said filtrate outlet are provided on the inner drum. Also included are a plurality of filtrate tubes in fluid communication with said filtrate outlet. The plurality of filtrate tubes connect the longitudinal fluid channels in fluid communication with the cavity, thereby establishing a fluid path between the longitudinal channels and the filtrate outlet.
A plate separates the cavity into an upper portion and a lower portion. The plate is fixedly attached to the housing, with the inner drum being free to rotate about the plate. An orifice controls a gas flow between the upper portion and the lower portion of the cavity and thereby controls gas flow through the filter from inside the housing.
The driven member comprises a shaft that is adapted to magnetically couple to an external prime mover. At least one permanent magnet is included on the shaft, and more preferably, a plurality of permanent magnets are disposed in a spaced-apart array around an end of the shaft. In one embodiment a magnetic material (such as iron), rather than a magnet, is incorporated on the shaft.
The device can include an external coupling hub adapted to be drivingly connected to a prime mover. In this case, the driven member is magnetically coupled to the external hub, which preferably includes at least one magnet or at least one electromagnet.
The means to remove the filter cake preferably comprises a generally sharp edge and has a length that is substantially equal to that of said inner drum, being formed as an integral part of the housing. Other means to remove the filter cake from the rotating drum include serpentine belts, bursts of compressed gas or air, or an additional, smaller rotating drum (roller discharge).
In some applications, the device must be sterile. For such applications, the housing is fabricated from a material that can be readily sterilized, e.g., using ethylene oxide, steam, gamma radiation, electron beam radiation, or light.
Another aspect of the present invention is directed to a method for using a disposable rotary drum filter to process a batch of slurry. The method includes steps generally consistent with the functions of the elements included in the apparatus described above.